Various material processing applications require use of a laser beam with a relatively flat top intensity profile, instead of a more traditional Gaussian intensity profile. These applications can include, for example, the drilling of holes in printed circuit boards with very little taper to the sidewalls of the holes, and the processing of glass or ceramics, which can include steps such as annealing, cutting, and fusing. Consequently, various techniques have been developed for obtaining beams with substantially flat top intensity profiles in line, square, or round shape configurations. One approach is to utilize the center portion of a Gaussian laser beam. This approach can be problematic, however, as a significant amount of power is lost due to the discarding of the remaining portion of the beam. The lost power cannot simply be compensated for by increasing the power of the laser, however, as the stability of the beam amplitude becomes increasingly more difficult to sustain as the laser power is increased. Further, such an approach can be expensive, as only a portion of the power will be used. Higher power lasers also tend to have larger power variations with time, as well as larger output wavelength variation over time.
Another approach to providing a flat top laser beam is to utilize diffractive optics. Such an approach is well known in the art, but has not been demonstrated to have better than a 10% amplitude variation across the intensity profile of the beam. For many applications requiring a flat top beam, a variation of 10% in intensity across the top of the profile is not acceptable. Further, such an approach again requires a high power laser beam, and it is well known that the output wavelengths and the gas discharges associated with high power lasers, such as lasers on the order of 500 W to 600 W, tend to have varying wavelengths over time, and numerous small discharge “hot spots” within the beam give rise to numerous amplitude variations (i.e., noise) in the output beams under continuous wave (CW) operation. The tendency to form arcs in CW discharges of large cross-sectional area slab lasers is one reason why slab lasers are normally operated under pulsed conditions. While operating a slab laser at a lower than normal gas pressure has been shown to improve the behavior of CW discharges, such an approach results in reduced output power for a given laser size.